How long can pre made "Saltwater" sit?

So I bought a 20L tank w/lid,heater, powerhead and air stone. How long will this pre mixed water be good for? I see you can buy pre mixed water off the shelf so im assuming it can last a while. Also do I need to run the powerhead, heater and air pump all the time or a few hours before I make do my water change?

Specs:
20 Long tank
1300 GPH powerhead
150 watt heater
12 inch air stone

Here is my setup for holding water.

 

Following

 

the SW you make yourself i wouldn't let set for more then a week MAX because it start to separate and leave mineral deposit behind. Unless its being mixed actively with a power head so its not just setting there.

 

Even if it's mixing non stop?

 

sorry i edited it as you where posting this. I think if its mixing non-stop you can go a bit longer but wouldnt push it. How much water are you trying to make up at once? I make about 30 gallons let it mix for 2 days then do a water change on all of it.

 

I wanted to push a month for all 20 gallons. Do 5 gallons here and there. Maybe even 5 gallons a week if need be.

 

i wouldn't let mixed water set around for a month that is a bit long might start growing bacteria from the actual water. Get a trash can and try to do you water changes that way you should be safe filling it up with 10gallons of water mix it up had it keep mixing then do a water change.

 

Well this just ruined my day! Well I'll make small amount when needed and I'll have a QT if ever needed.

Iv heard people making large amounts of RO water and storing them for months. I figured it would be the same with saltwater.

 

Same here...

 

if you have a sealed RO container sure you can do that but with the air and everything else being able to get to your made saltwater the bacteria might start growing you can try it but im betting it wont go over so well.